Backlight light guide

ABSTRACT

A backlight light guide comprises a generally planar structure having a light guide layer having a surface that includes a spaced array of extractors, a backside reflective layer, a low refractive index coupling layer disposed on an opposite major surface of the light guide layer, and a reflective polarizer to provide recycling of unused light.

THE FIELD OF THE INVENTION

The present invention relates generally to a lighting system and morespecifically to a backlight light guide for a display.

BACKGROUND OF THE INVENTION

Light guides are used in conjunction with light sources, such as lightemitting diodes (LEDs), for a wide variety of lighting applications. Inone particular application, light guides are commonly used to provideillumination for LCD displays. The light source(s) typically emit lightinto the light guide, particularly in cases where a very thin profilebacklight is desired, as in laptop computer displays. The light guide isa clear, solid, and relatively thin plate whose length and widthdimensions are on the order of the backlight output area. The lightguide uses total internal reflection (TIR) to transport or guide lightfrom the edge-mounted lamps across the entire length or width of thelight guide to the opposite edge of the backlight, and a non-uniformpattern of localized extraction structures is provided on a surface ofthe light guide to redirect some of this guided light out of the lightguide toward the output area of the backlight. Such backlights typicallyalso include light management films, such as a reflective materialdisposed behind or below the light guide, and a reflective polarizingfilm and prismatic brightness enhancement film(s) (BEF) disposed infront of or above the light guide, to increase on-axis brightness.

Since most commonly used light sources such as LEDs have a relativelylarge height and range of emission angles, the thickness of the lightguide is usually correspondingly thick to efficiently couple light. Aconventional illuminating device for a liquid crystal display isdescribed in US Publication No. 2009/0316431. Conventional illuminationdevices couple light from a source to a planar light guide. The lightguide typically is about the same height as the source, since reducingthe height of the light guide will reduce the coupling efficiency fromthe light source to the light guide.

A significant disadvantage of typical film or plate light guides,however, is the mis-match between the small aspect ratio of LEDs and thevery high aspect ratio of light guides. LEDs have a typical aspect ratioof about 1:1 to about 4:1, whereas edge light guides can have an aspectratio from about 20:1 to as much as about 100:1 or more. This mis-matchusually results in the light in the light guide having a much higheretendue, also referred to as throughput, than the light emitted from theLEDs. This high etendue in turn results in an increased thickness beingrequired for the light guide, as well as the light guide requiring airinterfaces on one or more of the faces. As a result, the light guide maybe thicker than the liquid crystal display module, and the airinterfaces may limit certain applications, such as touch and hapticapplications.

SUMMARY

In an aspect of the invention, a backlight light guide comprises agenerally planar structure having a light guide layer having a surfacethat includes a spaced array of extractors, a backside reflective layer,a low refractive index coupling layer disposed on an opposite majorsurface of the light guide layer, and a reflective polarizer to providerecycling of unused light.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description that follows moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are better understood with reference to thefollowing drawings. The elements of the drawings are not necessarily toscale relative to each other.

FIG. 1 is an isometric view of an exemplary backlight system accordingto an aspect of the invention.

FIG. 2A is an isometric partial view of a converter unit of a backlightsystem according to another aspect of the invention.

FIG. 2B is an isometric partial view of a diverting section of aconverter unit of a backlight system according to another aspect of theinvention.

FIG. 2C is another isometric view of a diverting section of a converterunit of a backlight system according to another aspect of the invention.

FIG. 2D is an isometric view of an anamorphic light guide section of aconverter unit of a backlight system according to another aspect of theinvention.

FIG. 2E is another isometric (bottom side) view of an anamorphic lightguide section of a converter unit of a backlight system according toanother aspect of the invention.

FIG. 2F is a side view of an anamorphic light guide section of aconverter unit of a backlight system according to another aspect of theinvention.

FIG. 2G is an isometric view of a coupling element portion of abacklight system according to another aspect of the invention.

FIG. 2H is another isometric view of a coupling element portion of abacklight system according to another aspect of the invention.

FIG. 2I is another isometric view of a converter unit of a backlightsystem according to another aspect of the invention.

FIG. 2J is an isometric view of an alternative converter unit of abacklight system according to another aspect of the invention.

FIG. 2K is a front view of an input face of another alternativeconverter unit of a backlight system according to another aspect of theinvention.

FIG. 2L is a front view of an output face of the alternative converterunit of FIG. 2K.

FIG. 3A is an illustration of the cross section of an exemplary lightbeam entering a converter unit according to an aspect of the inventionhaving an aspect ratio (X:Y) of from about 1:1 to about 2:1.

FIG. 3B is an illustration of the cross section of an exemplary lightbeam exiting a converter unit according to an aspect of the inventionhaving an aspect ratio (X:Y) of about 50:1.

FIG. 4 is an isometric view of an exemplary light source unit accordingto another aspect of the invention.

FIG. 5A is an isometric partial view of an exemplary backlight lightguide according to another aspect of the invention.

FIG. 5B is a schematic view of an exemplary backlight light guide unitaccording to another aspect of the invention.

FIG. 6A is an isometric view of an exemplary extraction elementaccording to another aspect of the invention.

FIG. 6B is a top view of an exemplary extractor layer of a backlightlight guide unit according to another aspect of the invention.

FIGS. 7A-7F are several views illustrating an exemplary process to forma backlight system and/or components thereof according to anotherembodiment of the invention.

FIG. 8 is an isometric view of a mold used to form a backlight lightguide with extraction features according to another aspect of theinvention.

FIGS. 9A-9F are several views illustrating an exemplary process to forma mold having extraction features according to another embodiment of theinvention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “forward,” “trailing,” etc., isused with reference to the orientation of the Figure(s) being described.Because components of embodiments of the present invention can bepositioned in a number of different orientations, the directionalterminology is used for purposes of illustration and is in no waylimiting. It is to be understood that other embodiments may be utilizedand structural or logical changes may be made without departing from thescope of the present invention. The following detailed description,therefore, is not to be taken in a limiting sense, and the scope of thepresent invention is defined by the appended claims.

The present invention is directed to a lighting system and morespecifically to a backlight system having an anamorphic light guide thatprovides an efficient lighting system for a display. The backlightsystem and its components, taken together or separately, are designed toprovide a highly efficient lighting system with low etendue. In thismanner, the number of overall components can be reduced and the need forair spaces can be eliminated, providing the opportunity for pressuresensing touch displays and haptics. The backlight system has severaladvantages, including being thinner, allowing lamination with anoptically clear adhesive (OCA), and eliminating or reducing the need forangular enhancement films.

FIG. 1 shows an isometric view of an exemplary backlight system 10 thatcan be used to illuminate a display (not shown), such as an LCD.Backlight system 10 includes a light source unit 100, a converter unit200, and a backlight light guide unit 300. Light source unit 100, shownin more detail in FIG. 4, provides a source of light for the backlightsystem 10. Converter unit 200, shown in more detail in FIGS. 2A-2I,includes an anamorphic light guide that guides the light from lightsource unit 100 into backlight light guide unit 300. Backlight lightguide unit 300, shown in further detail in FIGS. 5A-5B, includes abacklight light guide having a plurality of extraction features toprovide output light to a display, such as an LCD display. This outputlight has good uniformity. In addition, the system efficiently coupleslight from the light source to the display and provides output lightthat can be partially collimated in at least one axis. As such, theexemplary backlight system 10 can be used as part of a great number ofdevices and applications, such as transmissive, transflective, andreflective LCDs (laptops, tablets, cell phones, e-readers, etc.),cholesteric, MEMS, and liquid paper devices, signage and conformablegraphics, and indicators, such as vehicular displays.

Each of these components will now be described in greater detail. It isnoted that each of these components 100, 200, and 300 can be utilizedwith the other components of the exemplary backlight system of FIG. 1 orwith conventional backlight system components.

Regarding the converter unit 200, as shown in FIGS. 2A-2I, the converterunit 200 includes an anamorphic light guide 210 having an input face212, a diverting section 250, and an orthogonal light confining face 214that corresponds to the exit plane of the light exiting the converterunit 200. The converter unit 200 converts light emitted from the lightsource 100, which has an aspect ratio of less than about 10:1, such asabout 1:1 to about 1:2, into a line-shaped output light beam, having anaspect ratio greater than 10:1, such as an aspect ratio of about atleast 20:1, preferably about at least 50:1, or preferably about at least100:1. FIG. 3A shows an illustration of the cross section of anexemplary light beam 262 entering the converter unit 200 having anaspect ratio (X:Y) of about 1:1. FIG. 3B shows an illustration of thecross section of an exemplary light beam 264 exiting the converter unit200 having an aspect ratio (X:Y) of about 50:1. In one preferred aspect,the converter unit converts light emitted from the light source into aline-shaped output light beam having an aspect ratio greater than thelight source aspect ratio by at least a factor of four.

Input face 212 receives light from light source unit 100, described infurther detail below. Light is passed through the converter unit 200into a coupler 280 (which can be separate from or part of converter unit200), also described in further detail below, or alternatively, directlyinto backlight light guide unit 300. In one aspect, such as is shown inFIGS. 2D, 2E, and 2F, the light guide 210 is a generally rectilinearstructure having input surface 212, top surface 213, orthogonal surface214, opposite orthogonal surface 216, bottom surface 215, and endsurface 217. Surface 215 comprises a stepped surface, such that theheight of light guide 210 decreases along the length L from surface 212(having a height=h1) to opposite, end surface 217 (having a height=h2,where h2<<h1). In one example, for mobile unit backlight applications,h1 can be about 1 mm, the width can be about 2 mm, and L can be about 50mm to about 150 mm. In another example, for television and larger sizedisplay applications, h1 can be about 5 mm, the width can be about 10mm, and L can be from about 500 mm to about 1000 mm.

In one aspect, top surface 213 is approximately orthogonal with respectto input surface 212 and the bottom surface 215 includes a plurality ofsloping steps, with each sloping step parallel to the top surface 213.Thus, the light guide 210 can be a generally rectilinear, stepped, andsloped structure and can be formed from an optically clear material suchas a polymer (e.g., polycarbonate) or glass.

In addition, the light guide 210 can include a diverting section 250,that can include a plurality of diverting elements (also referred toherein as diverters) 251 a, 251 b, etc. (see FIGS. 2B and 2C), whereeach diverting portion changes the direction of the light byapproximately 90°. Depending on the size of the backlight light guideunit 300, the number of diverter elements can range from a few (3 or 4)to 20 or more. The diverting elements 251 a, 251 b, etc. can beintegrally formed as part of light guide 210 or they can be separatelyformed then attached to bottom surface 215 (see e.g., FIG. 2E) of thelight guide 210 using an appropriate adhesive or bonding material, suchas an optically clear adhesive.

In one aspect, each diverter comprises a coupled or decoupled input face252, a reflecting face 256 (e.g., faces 256 a, 256 b, etc. shown in FIG.2B) that changes the light direction by approximately 90°, and an outputface 254 that is coupled or decoupled to a coupling element 280 or thebacklight light guide unit 300. Each diverting portion is thin (relativeto the size of input face 212), such that each diverter input facecaptures only a segment of the incoming light and reflects that lightsegment towards the coupler 280/backlight light guide unit 300. Forexample, each diverter element can have a thickness of about 30 μm to200 μm, preferably about 50 μm. Thus, in one aspect, each divertingelement is configured as a generally planar right angle prism. As such,in one aspect, the height of the input surface 212 is approximatelyequal to the sum of heights of all of the diverting structures.

Each diverting element 251 a, 251 b, etc., may have a mirrored or TIR45° facet that reflects the incoming light by about a 90° angle. Lightis captured within each diverter, as the major faces of the diverter,top face 258 and bottom face 259, are each bounded by a lower indexmaterial. For example, bottom face 259 is bounded by air, while top face258 can be bounded by an optically clear adhesive, having a lower index(e.g., 1.49) than the index of refraction of the light guide 210.Alternatively, there may be a low index coating applied to eithersurface 215 or to surface 258, or both, and the surfaces coupled to eachother. Similarly, surfaces 213 and 259 may be coated with a low indexmaterial to allow the material to be bonded to other elements in thedisplay. Suitable low index coatings include silica and magnesiumfluoride. In another alternative aspect, the anamorphic light guide 210may be formed from a material with a lower refractive index than thematerial used to form the diverter 250. In yet another alternativeaspect, the refractive index of the anamorphic light guide may besimilar to the refractive index of the diverting element, without a lowindex material disposed between the two, and the light guide may have athickness less than the height h1 of the input face of the anamorphiclight guide, but greater than the thickness of the diverting section250.

As shown in FIG. 2B, a first input light segment 262 a is captured bydiverting element 251 a. The input light segment is totally internallyreflected within diverting element 251 a and directed off angledreflecting surface 256 a towards output face 254. The input lightsegment 262 a emerges the diverting element as output light segment 264a. Similarly, a second light segment 262 b is captured by divertingelement 251 b, which is axially spaced downstream from diverting element251 a at a height slightly raised above the height of diverting element251 a. The input light segment is totally internally reflected withindiverting element 251 b and directed off angled reflecting surface 256 btowards output face 254. The input light segment 262 b emerges thediverting element as output light segment 264 b. In a similar manner,each subsequent diverting element captures a segment of the input lightand redirects that light segment towards the coupler 280/backlight lightguide unit 300. Thus, the output light segments 264 a, 264 b, etc. forma line shaped beam having a high aspect ratio of at least 20:1 orgreater.

Reflecting surfaces 256 a, 256 b, etc., can be flat or curved surfaces.In addition, in some aspects, the reflecting surfaces 256 a, 256 b, etc.can be coated with a reflective coating. For example, the reflectingsurfaces 256 a, 256 b, etc. can be coated with a metal or a dielectriclayered coating. Alternatively, the reflecting surfaces 256 a, 256 b,etc. can be simply polished to totally internally reflect (TIR) light.

In construction, for converter units that comprise separately formedlight guides and diverting sections, the diverting section 250 can bemated to the light guide 210 on bottom surface 215 using an opticallyclear adhesive or low index bonding material. In this aspect, divertingelement input surface 252 a (see FIG. 2B) can be mated with bottom stepsurface 215 a (see FIG. 2E), next diverting element input surface 252 bcan be mated with next bottom step surface 215 b, and so forth.According to alternative aspects, the input face(s) of the diverter(s)250 may be either optically coupled or decoupled from the light guide210. Optically coupling the diverter can be more efficient due toreducing Fresnel reflections, but may cause losses with diverters with a45° facet due to errant paths for the light beam. Therefore,alternatively, when utilizing diverting elements having a 45° facet, theinput face may be decoupled from the light guide 210. Similarly, theoutput face of the diverter elements, e.g., face 254 (shown in FIG. 2B)may be coupled or decoupled from the input face of the coupler280/backlight light guide unit 300.

In alternative aspects of the invention, the converter unit 200 can havealternative constructions. For example, as shown in FIG. 2J, converterunit 200′ can comprise a stack of films 205 a-205 g. The films can bestacked one on top of the other, with an intervening layer of anoptically clear adhesive or a low index of refraction coating (notshown). Each film can have reflector surface, such as surface 264 g,comprising a mirrored or TIR 45° facet that reflects the incoming light(e.g., rays 262 a-262 g) that enters converter unit 200′ via input face212′ by about a 90° angle, such that is diverted and output as rays 264a-264 g. As shown in FIG. 2J, the reflecting surfaces are sequentially,axially spaced apart from each other. Alternatively, each layer of filmcan comprise a series of etched lines formed therein, such as roundedbends, that create diverting channels within each layer of film tochannel the incoming light towards the coupler 280/backlight light guideunit 300. In addition, the efficiency of curved light guides can beincreased by optically coupling the output face of the diverting section250 to the input face of the coupler 280/backlight light guide unit 300.

This alternative construction maintains the uniformity of the source inat least one direction—the light source may have a non-uniform intensityof light when illuminating a light guide, and the stack of filmsmaintain the distribution in one axis. The films also allow light to bedistributed in the other axis to promote uniformity of the lightilluminating the backlight light guide unit 300. As mentionedpreviously, a low index coating may be interposed between film layers tomaintain isolation of light from one film layer to another. Depending onthe requirements of the overall display system, this construction canadd to overall thickness and can reduce coupling efficiency.

In another alternative embodiment, as shown in FIGS. 2K and 2L, analternative converter unit can comprise an array of optical fibers. Inthis example, input face 212″ is shown in FIG. 2K comprising an array ofN by M fibers. This input face 212″ is configured to receive light fromthe light source unit having an aspect ratio of about 1:1. The outputface 254″ comprises a single row of N×M fibers, thus providing outputlight at an aspect ratio of about at least 20:1.

Thus, the converter unit 200 can comprise a rigid or flexible body, witha tapered or non-tapered shape that can convert the aspect ratio of thesource by over an order of magnitude.

The above-described converter units are configured to convert theformat, or aspect ratio, of the incoming light source into a line. Thisconstruction also substantially preserves the etendue of the lightsource.

Source light can be provided by any number of source types, but a morepreferred source is an LED light source.

FIG. 4 shows an exemplary light source unit 100. Light source unit 100can include a single LED, such as LED 110, two LEDs, or more LEDs,depending on the type of display being illuminated. The output of theLED(s) 110 may be coupled to the converter unit 200 using one or morecompound parabolic concentrators 105, lenses (not shown) or acombination thereof. Of course, in alternative embodiments, a lens or amultiple lens system can be utilized to collect and collimate the outputof the LED(s).

As shown in FIG. 4, the LED(s) 110 can be mounted on the one or moreentrance apertures 102 a-102 d. While FIG. 4 shows a compound parabolicconcentrator (CPC) 105 that is configured to collect and concentratelight from four LEDs, in other aspects of the invention, the CPC 105 cancollect and concentrate light from fewer or greater numbers of LEDs. Theinterior portion of the CPC 105 can be hollow and constructed in thesame manner as that of a conventional CPC. The LED light is output fromexit aperture 104.

In this regard, “light emitting diode” or “LED” refers to a diode thatemits light, whether visible, ultraviolet, or infrared, where theemitted light will have a peak wavelength in a range from about 430 to700 nm. The term LED includes incoherent light sources that are encasedor encapsulated semiconductor devices marketed as “LEDs”, whether of theconventional or super radiant variety, as well as coherent semiconductordevices such as laser diodes, including but not limited to verticalcavity surface emitting lasers (VCSELs). An “LED die” is an LED in itsmost basic form, i.e., in the form of an individual component or chipmade by semiconductor processing procedures. For example, the LED diemay be formed from a combination of one or more Group III elements andof one or more Group V elements (III-V semiconductor). Examples ofsuitable III-V semiconductor materials include nitrides, such as galliumnitride, and phosphides, such as indium gallium phosphide. Other typesof III-V materials can also be used, as well as materials from othergroups of the periodic table. The component or chip can includeelectrical contacts suitable for application of power to energize thedevice. Examples include wire bonding, tape automated bonding (TAB), orflip-chip bonding. The individual layers and other functional elementsof the component or chip are typically formed on the wafer scale, andthe finished wafer can then be diced into individual piece parts toyield a multiplicity of LED dies. The LED die may be configured forsurface mount, chip-on-board, or other known mounting configurations.Some packaged LEDs are made by forming a polymer encapsulant over an LEDdie and an associated reflector cup. The LED may be grown on one ofseveral substrates. For example, GaN LEDs may be grown by epitaxy onsapphire, silicon, and gallium nitride. An “LED” for purposes of thisapplication should also be considered to include organic light emittingdiodes, commonly referred to as OLEDs.

In one aspect of the invention, the LED(s) 110 may be made from an arrayof two or more different color LEDs, for example red-green-blue (RGB)LEDs (e.g., a red LED in combination with a green LED in combinationwith a blue LED), or, alternatively, a combination of a red LED with acyan LED. In another aspect, the LED(s) 110 may comprise one or moreremote phosphor LEDs, such as those described in U.S. Pat. No.7,091,653. In this manner, an appropriate balance of blue and yellowlight can create white light output to the backlight light guide unit300.

In another aspect, a blue GaN LED, a YAG phosphor, and collimatingoptical systems such as lenses and compound parabolic concentrators canbe utilized as light source unit 100. An additional illuminator having adifferent color output can also be used in combination.

With the design of the system of the present invention, the light source100 can utilize very high brightness and efficient LEDs, mix and matchdifferent discrete colors, and utilize remote phosphor-based LEDs. Atthe same time, the efficient conversion of light, through thepreservation of etendue, can eliminate the need for a large number ofLEDs to be utilized.

The light sources may produce homogenous colors, such as that from aphosphor converted LED, or may be a combination of colors. For example,the LEDs may be a combination of a blue LED with a green-emittingphosphor and a red emitting AlInGaP LED. The combination of theanamorphic light guide and the diverters has been found to providesufficient path length for the light emitted from the LEDs toeffectively mix the colors before entering the backlight light guideunit.

Referring back to FIGS. 2A-2C, in one exemplary aspect of the invention,light exiting the anamorphic light guide 210 can be received by acoupler 280 that links the converter unit 200 and the backlight lightguide unit 300. The coupler 280 can be part of or separate from theconverter unit 200. In this example, the coupler 280 will be describedas being part of the converter unit 200. For example, as shown in FIGS.2B and 2C, coupler 280 can be integrally formed with diverting elements250 in a single piece construction.

As shown in more detail in FIGS. 2G and 2H, coupler 280 comprises agenerally rectilinear body having a partial stepped profile along oneside (e.g., input face 282) to receive the output of the anamorphiclight guide and a line-shaped profile along the opposite side (e.g.output face 284) to couple light into the generally planar backlightlight guide of unit 300. In particular, the partial stepped input face282 of coupler 280 can comprise a series of faces (e.g., faces 282 a-282e, each stepped in height by an amount corresponding to the thickness ofeach diverting element) that correspond to and register with thearrangement of output faces 254 of the diverting element 250. On theopposite side of coupler 280, output face 284 has a substantiallyline-shaped face and a thickness or height (h3) to substantially matchthe thickness of the backlight light guide portion (see light guide 310in FIG. 5A) of the backlight light guide unit 300. Light is guidedwithin coupler 280 via TIR. Thus, the coupler 280 is configured tosubstantially preserve the etendue of the anamorphic light guide, whilecorrecting for the disparity in shape from a profile of thin, steppedfaces to a generally line-shaped face.

As mentioned above, in one aspect of the invention, coupler 280 isintegrally formed with diverter 250. In this aspect, the diverter 250and coupler 280 may be made from a continuous molded article. Suitablematerials of construction include acrylic resins, includingpolymethylmethacrylate (PMMA), curable acrylic resins, polystryrene,polycarbonate, polyesters, and silicones. Alternatively, coupler 280 canbe formed using a cut strip of polymer film or by a cast and cureprocess.

In some cases, the area of the input to the planar light guide can besubstantially larger than the output of the anamorphic light guide (byapproximately 2×), thus the thickness of the planar light guide will bethicker than is needed from the perspective of etendue.

The etendue of the system may be preserved by matching the area of thebacklight light guide to the output of the coupler 280. This matchingmay be done by either one or a combination of reducing the thickness ofthe backlight light guide to make it thinner than conventional backlightlight guides or by tapering the profile of the coupler 280 such thatoutput face 284 has a greater thickness than that of partial steppedinput face 282. In some alternative aspects, the taper may be linear orthe taper may be non-linear in at least one axis. A suitable non-linearprofile may include a parabola.

A low refractive index layer can be disposed between the anamorphiclight guide 210 and the diverter 250. The low refractive index layer maycomprise a polymer coating or a coating applied by physical vapor orchemical vapor deposition. In a preferred aspect, the low index coatingwill have low scatter. Suitable coatings can include silica, SiO₂, andMgF₂.

In an aspect of the invention, light exiting coupler 280 entersbacklight light guide unit 300, which further directs light towards adisplay. As shown in FIG. 5A, the backlight light guide unit 300includes a generally planar structure having one or more layers. In oneaspect, backlight light guide unit 300 includes a main or first(central) layer 310 disposed between a layer 330, and layer 320. Thefirst (central) layer 310 can comprise a high index polymer layer, e.g.,polycarbonate, polystyrene, or cured phenyl acrylates, that serves asthe main backlight light guide. An array of extractors 315 (see e.g.,FIGS. 6A, 6B) that direct light towards the viewing side of the device(in this example, towards the display panel), can be disposed on abottom surface of layer 310. The second layer 330 can comprise a lowerrefractive index material, such as an optically clear adhesive (OCA). Asan adhesive, in some aspects, layer 330 can be attached to an LCD moduleor an intermediate film, such as a brightness enhancement film. Thelayer 320 can comprise a reflective surface to act as a backside mirror.

Input light enters the first (central) layer 310 of backlight lightguide unit 300 in the direction of arrow 305. In some aspects, layer 310can have an index of refraction of about 1.55. Light can be deflected bythe extractors in the direction of arrow 307 to provide illumination forthe display panel (not shown). As the light exiting the converter unit200 has a low etendue (e.g., less than 5), that light is well collimatedentering layer 310. As a result, the index of refraction of layer 330 isnot required to be substantially lower than that of layer 310 tomaintain an effective waveguide structure. For example, in an aspect ofthe invention, layer 330 has an index of refraction of about 1.49. Inother words, with the light guide design described herein, an airboundary on either side of layer 310 is not required to achieve aneffective waveguide structure. In addition, the thickness of layer 310can be substantially reduced (as compared to conventional backlightsystems).

In one aspect, first (central) layer 310 comprises a material having athickness of about 50 μm to about 500 μm. A preferred thickness may bebased on the height of the collimating optics (e.g., CPC) used in thelight source, where the thickness of layer 310 can be about ½the heightof the collimating optics. Layer 310 preferably has a generallyrectangular shape, although in alternative aspects, layer 310 can bewedge-shaped. The reduced thickness of layer 310 represents asubstantial improvement over conventional backlight systems and is aboutan order of magnitude less in size (thickness) than the size (e.g.,height) of the LED light source. In conventional backlight systems, themain backlight light guide is typically surrounded on both major sidesby an air surface or interface, as the widest range of TIR occurs ingeneral when air acts as a light guide cladding. However, air claddingis not acceptable when the light guide is to be in physical contact withstructural elements on one or both major sides of the backlight lightguide. Previous approaches to this configuration are not optimal. Theseprevious approaches include accepting greater light losses due to poorerTIR collection angle range and increasing the thickness of the backlightlight guide to accept an increased height of a collimated light beam.These approaches fail to meet the demands of improved power efficiencyand more compact systems.

The light source unit 100 and converter unit 200 described abovesubstantially preserve etendue and produce light having a high aspectratio (of 20:1 or greater) and good collimation. In a preferred aspect,the light emitted by the LED is collimated such that at least 25% of thelight emitted by the LED is contained within a cone with a half-angle ofno more that about 15°, more preferably within a cone of no more thanabout 10°. As a result, the thickness of the backlight light guide unit300 can be substantially reduced (e.g., by about 2× or more). Inaddition, the low scatter of the entering light means that air claddingis not required and overall device thickness can be even furtherreduced.

In another aspect, such as is shown in FIG. 5B, the backlight lightguide unit 300 includes a generally planar structure having thefollowing layers: a light guide layer 310 having a surface that includesa spaced array of extractors, a backside reflective layer 320, a lowrefractive index coupling layer 330 disposed on the opposite majorsurface of layer 310, a quarter wave film layer 340 to shift thepolarization of the incoming light by a quarter wavelength, and areflective polarizer 350 to provide recycling of unused light.

LCDs transmit one polarization of light. Since most light sources areunpolarized, the polarized transmission in conventional LCDs leads to asignificant loss of optical efficiency and increases the power usage ofthe display. In contrast, with the present design, such as shown in FIG.5B, a reflective polarizer 350 can increase display efficiency byreflecting the light polarization back into the backlight, allowing someof the light to be converted into the useful polarization state byquarter wave film layer 340.

According to alternate aspects of the invention, two classes ofapproaches can be used to convert the reflected polarized light into thedesired transmitted polarization. One approach is to use components inthe backlight that randomize the polarization of the light. Usingreflective polarizers with a scattering, lambertian-type reflector as abackside reflector, tends to depolarize the light. Suitable polarizationrandomizing reflective materials include metal coatings, dichroiccoatings, and combinations thereof on optically thick and birefringentpolymers such as polyethylene naphthalate and polyethylene terephthalate(PET). Semispecular reflectors may also be suitable, including orientedvoided PET films. This configuration creates more reflections in therecycling cavity, and may reduce efficiency. An advantage of this typeof reflector is reducing the number of optical components, such as thequarter-wave retarder.

A second approach, such as is shown in the aspect of the invention inFIG. 5B, is to use a specular (polarization-preserving) backsidereflective layer with a quarter-wave retarder disposed between thebackside reflective layer and the reflective polarizer. This secondsystem approach can be more efficient than the first approach, whichrandomizes the polarization state. In this second approach, the opticalcomponents disposed between the backside reflector layer and thereflective polarizer layer should have very low optical birefringence tomaximize output efficiency. Suitable polarization-preserving reflectorsinclude metalized and inorganic dichroic reflectors, and combinationsthereof disposed on a low birefringent material such aspolymethylmethacrylate (PMMA) and other amorphous polymers.

In addition, due to the low etendue of the light source unit 100 andconverter unit 200, the light passing through the quarter-wave filmtypically has a much narrower range of angles, which eliminates the needfor an expensive, broad use-angle range quarter-wave film.

Suitable materials for low refractive index layer 330 include SiO₂,MgF₂, silicone polymers, fluoropolymers, acrylics, and mixtures thereof.

A simulation was performed comparing a conventional backlight system(1), that utilizes a lambertian-type scattering backside reflectivelayer, to a backlight system (2) such as shown in FIG. 5B, where layer520 includes a 90% (reflective) specular reflector, layer 310 is formedfrom a transparent material with an array of extractors, a low index OCAlayer 330, a quarter-wave retarder layer 340, and a reflective polarizerlayer 350 (formed from APF, available from 3M Company). The reflector isbonded to the backside of the light guide, where, for this example, athin low index coating can be applied to the backside of the light guideand extractor array (e.g., a physical vapor deposited MgF₂ or silicacoating, followed by a coating of silver or aluminum). The backlightsystem (2) results in a more efficient system than backlight system (1),between 30% and 100% more efficient, depending on the usage of angulargain films. The greatest improvements in efficiency occur when thereference display uses angular gain films such as BEF film, availablefrom 3M Company. The angular gain films typically are positioned in thisexample between the top surface of the light guide and below thereflective polarizer. The simulated efficiency differences werecalculated using Lighttools software, available from Synopsis Corp.

FIG. 6A shows an exemplary extraction feature or extractor 312.Extractor 312 has a truncated pyramid or tooth-shape, with angled sides,that is designed to pick off light rays and deflect them at asubstantially 90° angle toward the display unit (not shown). While theinternal angles are shown in FIG. 6A as 45° angles, these internalangles may be lesser or greater angles, depending on the properties ofthe light being guided within backlight light guide 300. The extractoris formed in the light guide, and can use total internal reflection(TIR) to promote reflection, using air interfaces, and optionally incombination with dielectric thin film coatings, such as silica ormagnesium fluoride. The extractors can be formed in the light guidethrough microreplication, where, for example, a radiation cured resinreplicates a pattern on a metal tool surface onto a light guide film.Alternatively, the light guide can comprise a layer of glass. Theextractors can comprise areas of texture on the light guide plate, wherethe texture controllably scatters light passing through the light guideplate. Alternatively, the extractors may comprise geometric featuressuch as prisms. An example extraction feature is an array of prismsdistributed over the surface of the light guide. The distributionprovides the desired light output uniformity for the light guide. Theprisms may be arranged in a one-dimensional array of, for example,concave right-angle prisms, with each prism being about 1.5 μm inheight, 3 μm in length, and the width of the light guide, such thatlight is extracted from the light guide. The prisms may also be arrangedas a two-dimensioned array, with the length of each prism being, forexample, about 10 μm.

FIG. 6B shows a top view of extractor layer 315 showing the relativelywide spread spacing of each extraction unit. In this manner, eachextractor of the extractor pattern covers an extremely small area of thebacklight unit, thus promoting uniformity of the reflected light raystowards the display panel. In one aspect of the invention, the densityof extractors is about less than 20% of the area of the light guide. Inanother aspect, the density of extractors is about less than 10% of thearea of the light guide. As a result, light rays emitted by thebacklight system are in a relatively small range of angles, aconsequence of the low etendue design of the backlight system and theuse of the extractors described herein. This low density of extractorsallows the recycling polarizer arrangement of the embodiment of FIG. 5Bto operate more efficiently because fewer extractors leads to lessrandomization of the light polarization. An exemplary process forforming the extractor layer 315 is described in further detail below.

According to simulations performed by the investigators, the addition ofa reflective polarizer to a conventional backlight typically increasesbrightness by 50% to 70%. Using an example model system in LightToolsversion 7.2 (available from Synopys Inc., Mountain View, Calif., USA), aconventional BacklightSystems 3LEDBacklight shows a 72% increase inbrightness by adding a simulated APF film. For a system configuredsimilar to the embodiment such as shown in FIG. 5B, where layer 320includes a 90% specular reflector, layer 310 is formed from a polymericmaterial with an index of 1.58 and having an array of extractors, a lowindex OCA layer 330, and a quarter-wave retarder layer 340, the additionof a reflective polarizer layer 350 (formed from APF, available from 3Mcompany) results in a 93% increase in efficiency.

Thus, while conventional LCD backlights have a relatively low 60-70%gain using reflective polarizers, according to exemplary aspects of theinvention, the backlight system described herein can provide an 80-90%gain. According to other aspects, the backlight light guide can have alow density of extraction features, a highly reflective back surface,and a reflective polarizer. The backlight light guide may use prismaticextraction features, and may include a quarter wave retardance film. Thebacklight may also contain a non-depolarizing diffuser.

According to another aspect of the invention, several components of thebacklight unit described herein, including elements of the converterunit 200 and backlight light guide unit 300, can be formed using thefollowing process. FIGS. 7A-7F are used to help illustrate the process.

Overall, the exemplary process includes providing a cavity having atleast a first array of first optical elements and a second array ofsecond optical elements that have a different shape than the firstarray. For example, in one aspect, the first array of optical elementscan comprise the diverters and the second array of optical elements cancomprise the backlight light guide unit with extractors. In anotheraspect, the second array of optical elements can comprise the couplingelement. The process further includes filling the cavity with a curableresin. Another optical element, or secondary optical element, such asthe anamorphic light guide, can be applied to the curable resin inalignment with the first optical array. The resin can then be cured. Thecured assembly can then be removed from the cavity. In alternativeaspects, a molding tool, such as a surface of the coupling element, canbe applied to the same side of the first array as is the anamorphiclight guide. The exemplary process may be continuous, using molds thatare on a belt or a cylinder, semicontinuous, or batch.

In more detail, in FIG. 7A, a molding surface or mold 400 is shown (inpartial view) having a depressed area (e.g., cavity) with a negativeimage of desired optical element shapes. The negative image of thedesired optical element shapes may be any indented or bumped shape, orcombination of the two. The optical elements may be arrays or randomshapes, and may include, for example, aspheres, spherical shapes,prisms, torus shapes, and channels. Mold 400 can be prepared by diamondturning or flycutting a master form and electroforming the mold. Mold400 includes a section 420 that is configured to include the converterunit of the backlight guide (e.g., diverters, anamorphic light guide).For example, section 420 can include triangular shaped recesses. Section450 is configured to include the backlight light guide. For example,section 450 can have a construction such as shown in FIG. 8 (shown inexaggerated scale), with an array of extractors or extractor layer 415,similar to extractor layer 315 described above, formed on a bottomsurface 411 of the mold. A process that can be used to form the moldwith the array of extractors 415 is described in further detail below.

In one aspect, the mold 400 can be configured to form a backlight guidefor a mobile or handheld device. In other aspects, the process describedherein can be utilized to form a backlight system for a larger display,such as a tablet, computer, or television display.

Optionally, the molding surface can be coated with a release agent, or amaterial that has desired optical characteristics, such as a lower indexof refraction than a curable resin. This coating may remain with themold or adhere to the curable resin once cured. Examples of suchsuitable coatings include diamond like coatings, silicones, acrylates,fluoropolymers, and physically vapor deposited materials.

In FIG. 7B, the mold 400 is filled with a curable resin 455. The curableresin may be applied by dip coating, wire bar coating, doctor bladecoating, ink jet coating, roll coating, silk screen coating, or anyother coating process. The curable resin may be a single composition, ormay vary by the region on the molding surface. Suitable resins includeacrylates, silicones, epoxies, esters, vinyl compounds, and may include,for example acrylic acid, methyl methacylate, other monofunctionalacrylates, polyfunctional acrylates, dimethyl silicone, methyl-phenylsilicone, fluoroacrylates, and mixtures thereof.

In FIG. 7C, a secondary optical element, such as the anamorphic lightguide 210, can be placed onto the curable resin at section 420 inregistry with the array of optical elements (e.g., diverters). Thesecondary optical element may be applied to the surface of the resin inan atmospheric environment, or in an inert gas environment such asnitrogen, helium, argon, or carbon dioxide. The optical element or theresin or both may be heated to reduce resin viscosity, and reduce thetendency to entrain gas. A pressure below atmospheric may also be usedto reduce gas entrainment. The secondary optical element may be alignedwith fiducials, or the features on the mold, or may be passively alignedwith the features on the mold, or by a combination of the two or moreprocesses. The height may be adjusted to reduce the meniscus between theoptical element and the resin. The secondary optical element may beapplied at the same time as the secondary molding structure, or may theymay be applied separately.

In FIG. 7D, one or more removable secondary molds are applied to thesurface of the mold 400. In this example, secondary mold 422 is appliedto section 420 of mold 400. Secondary mold 422 is preferably transparentto curing radiation, such as e-beam radiation. The optional removablesecondary molds may be applied to some or all of the resin surface. Thesecondary molding surface may be aligned with fiducials, or the featureson the mold, or may be passively aligned with the features on the mold,or by a combination of the two or more processes. The height may beadjusted to reduce the meniscus between the molding surface and theresin. In this example, the mold 422 can be shaped so as to form thecoupling element (such as coupling element 280) after curing. Inaddition, optionally, a film can be applied to the surface of the mold400 to assist in the removal of the secondary mold(s) after curing.

After placement of the removable secondary mold, the resin 455 is cured.The resin may be cured using a thermal initiator or catalyst, bythermally driven condensation, a photoinitiator, or by other actinicradiation including electron beams, or a combination of one or more ofthese processes. In one aspect, resin 455 is cured by radiation, such ase-beam radiation, using a conventional curing process. While UV and/orother light beam curing methods can be employed. Using e-beam radiation,as opposed to a UV curing process, can reduce potential light absorptionissues.

In FIG. 7E, mold 400 is shown after curing, with the secondary mold 422removed.

In FIG. 7F, an optically clear adhesive film, with a liner, can beapplied to the top surface of the cured structure. The assembly, whichincludes the backlight light guide, coupling element, diverters, andanamorphic light guide can then be removed from the mold 400.

In addition, one or more surfaces of the cured resin structure and thesecondary optical elements may be post-processed. Suitable postprocesses includes being physical vapor coated with dielectric materialssuch as MgF₂, SiO₂, or Al₂O₃, or metals including aluminum or silver, orcombinations of dielectrics and metals. In one aspect, a suitablecombination includes a coating of a low index dielectric material suchas MgF₂ or SiO₂ followed by a coating of aluminum or silver. The lowrefractive index dielectric coating increases reflectivity at highangles, and is transparent at higher angles allowing the metal toeffectively reflect light.

After removal and/or post processing, the assembly can then be bonded toan upper or lower display surface (not shown).

Thus, the above process can be utilized to produce one or more elementsof the backlight light guide system 10 shown in FIG. 1.

As mentioned above, the backlight light guide includes an array ofextractors or extractor layer that redirects light toward the display ina uniform manner. FIGS. 9A-9F below illustrate an exemplary process thatcan be utilized. In this exemplary process, a surface is provided, wherean array of grooves are formed on the surface. The grooves are filledwith a polymer, which can be planarized. The polymer can be furtherdefined through a combination of patterned radiation and etching.Electroforming can be used to form a replica of the surface with theselected portions of the grooves, where the sides of the grooves canhave an angle of at least 45 degrees to the plane of the light guide.The resulting lightguide can have a major surface with light (lowdensity) extraction features (i.e., extractors), where the extractionfeatures occupy less than 10% of the area of the major surface.

FIG. 9A shows an isometric view of a substrate 401 having a series ofgrooves 403 formed in an upper surface 402 thereof. The substrate 401can be formed from a metal, such as copper or nickel, or alloys thereof.The grooves 403 can be formed using a conventional cutting process, suchas a diamond cutting process.

FIG. 9B shows substrate 401 coated with a polymer layer 404, which fillsthe grooves formed in the upper surface 402. The polymer layer 404 maycomprise any one or a mixture of materials. In one aspect, the polymerlayer 404 comprises a photoresist. Alternatively, polymer layer 404 maycomprise a polymer that is etchable, such as by a conventional e-beam,reactive ion, or similar etching process.

Optionally, the polymer-coated substrate can then be planarized, as isillustrated in FIG. 9C. With this step, polymer/resist can remain in thepreviously formed grooves, while the remaining polymer/resist is removedfrom the upper surface 402. Suitable conventional processes forplanarization include, for example, abrasive polishing and chemicalmechanical polishing (CMP). The planarization can increase thesmoothness of the completed light guide. Planarization may not berequired in some applications.

The polymer/resist (planarized or not) layer 404 can then be exposed topatterned radiation. Alternatively, the polymer layer 404 may be coveredwith a patterned etch barrier, and the polymer layer may be patternedthrough reactive ion etching. FIG. 9D shows the substrate 401 after theetching step is completed.

In some aspects, the etched faces of the extractor features intersectingthe major surface of the light guide may have a high angle and may benearly perpendicular to the major surface. A small deviation in theangle from normal can be utilized in some aspects to facilitatereleasing of the tool surfaces after electroforming. The etch surfaces407 are also preferably smooth and do not substantially scatter light.In some applications, it may be preferred to have the etched facesperpendicular, or even undercut. In some aspects, the etched faces havean angle of between 90 and 60 degrees from the major surface, morepreferably, the angle is between 85 and 60 degrees, and most preferablythe angle is between 80 and 70 degrees from the major surface.

The etched substrate can then be coated with a metal layer, andelectroformed with another metal, such as nickel, copper, or alloyscontaining nickel or copper, or both. FIG. 9E shows the completedelectroform 408 that is formed over substrate 401.

FIG. 9F shows the electroform 408 after separation from the substrate.Either the substrate 401 or the first electroform 408 may be used forsubsequent electroforming steps to form additional tools for cast andcure or injection molding operations.

Thus, the backlight system and components thereof described aboveprovide an efficient lighting system for a display. The backlight systemand its components, taken together or separately provide a highlyefficient lighting system with low etendue and a reduced number ofoverall components. With the backlight system described here the needfor air spaces can be eliminated, providing the opportunity for pressuresensing touch displays and haptics. The backlight system can be thinnerthan conventional backlights, allowing lamination with optically clearadhesive. In addition, the need for angular enhancement films iseliminated.

Although specific embodiments have been illustrated and described hereinfor purposes of description of the preferred embodiment, it will beappreciated by those of ordinary skill in the art that a wide variety ofalternate or equivalent implementations may be substituted for thespecific embodiments shown and described without departing from thescope of the present invention. Those with skill in the art will readilyappreciate that the present invention may be implemented in a very widevariety of embodiments. This application is intended to cover anyadaptations or variations of the embodiments discussed herein.

1. A backlight light guide, comprising: a a light guide layer having asurface that includes a spaced array of extractors, a backsidereflective layer, a low refractive index coupling layer disposed on anopposite major surface of the light guide layer, and a reflectivepolarizer to provide recycling of unused light.
 2. The backlight lightguide of claim 1, wherein the backside reflective layer comprises alambertian-type reflector.
 3. The backlight light guide of claim 1,further comprising a quarter wave film layer to shift the polarizationof the incoming light by a quarter wavelength.
 4. The backlight lightguide of claim 3, wherein the backside reflective layer comprises apolarization-preserving specular reflector.
 5. The backlight light guideof any of claims 1-4, wherein each extractor comprises a truncatedpyramid or tooth-shape, with angled sides.
 6. The backlight light guideof claim 1, wherein a height of each extractor is about 5 μm to about 10μm.
 7. The backlight light guide of claim 1, wherein a surface of eachextractor is coated with a reflective metal layer.
 8. The backlightlight guide of claim 1, wherein the density of extractors is about lessthan 20% of the area of the backlight light guide.
 9. The backlightlight guide of claim 1, wherein the light guide layer comprises agenerally rectangular structure formed from a high index polymer. 10.The backlight light guide of claim 9, wherein the high index polymercomprises at least one of polycarbonate, polystyrene, and cured phenylacrylate.
 11. The backlight light guide of claim 1, wherein there is noair boundary directly on either major side of the light guide layer. 12.The backlight light guide of claim 1, wherein the backside reflectivelayer comprises a polarization randomizing reflective material.
 13. Thebacklight light guide of claim 4, wherein the polarization-preservingspecular reflector comprises at least one of a metalized reflector andan inorganic dichroic reflector.
 14. The backlight light guide of claim1, wherein the low refractive index coupling layer comprises one ofSiO₂, MgF₂, silicone polymers, fluoropolymers, acrylics, and mixturesthereof.
 15. The backlight light guide of claim 1, wherein the array ofextractors are formed through microreplication.
 16. The backlight lightguide of claim 1, wherein the backlight light guide provides a gain ofat least 80%.
 17. The backlight light guide of claim 1, furthercomprising a non-depolarizing diffuser.